Finite-element modeling (FEM) was used to simulate processing by high-pressure torsion under quasi-constrained conditions using three different material conditions: strain-hardening, perfect-plastic, and flow-softening. The results show there is a tendency for flow localization during processing and this becomes more obvious during the processing of perfect-plastic, and flow-softening materials or when processing samples having high thickness to diameter ratios. The analysis demonstrates the effect of the material condition, the disk aspect ratio and the effect of friction between the disks and the anvil walls. It is demonstrated that the predictions from FEM correlate well with published experimental results.